1. Field Of The Invention
This invention relates to new ceramic electrostrictive compositions, a process for the preparation of the compositions into an electrostrictvie material and specific applications for the electrostrictive material.
2. Description Of The Prior Art
The electrostrictive effect relates to a lattice phenomenon in the crystal lattice of materials. Particularly, the electrostrictive effect comes from the direct attraction and repulsion of anions and cations in a crystal lattice, said attraction and repulsion resulting from the application of an external electric field. The attraction and/or repulsion in the crystal lattice results in a physical distortion of the lattice. This lattice distortion causes a displacement or strain in the material. Stated quantitatively, the strain in an electrictive material is proportional to the electrostrictive coefficient multiplied by the square of the electric field. The relationship between strain and polarization and the relationship between strain and dielectric constant are shown below in Equations 1a and 1b, wherein S=strain, Q=electrostrictive coefficient, K=dielectric constant (relative permittivity at the applied field), .SIGMA.o=permittivity of free space, P=polarization and E=applied electric field: EQU S=QP.sup.2 [Equation 1a] EQU S=Q(K.SIGMA.o).sup.2 E.sup.2 [Equation 1b]
The electrostrictive effect should not be confused with the piezoelectric effect, or more particularly, the converse piezoelectric effect. In the converse piezoelectric effect the amount of strain which occurs in a piezoelectric material is equal to the piezoelectric strain coefficient multiplied by the electric field. The relationship between strain and applied electric field is shown below in Equation 2, wherein S=strain, d=piezoelectric strain coefficient and E=applied electric field. EQU S=dE [Equation 2]
Moreover, a single material can exhibit both the piezoelectric effect and the electrostrictive effect. To exemplify how a material can show both the piezoelectric and the electrostrictive effect, reference is made to FIG. 1.
FIG. 1 shows a simplified curve for a material which exhibits both a piezoelectric effect and an electrostrictive effect. Particularly, FIG. 1 shows the relationship between dielectric constant and temperature for such a material. The temperature at which the dielectric constant is the highest, at a specific frequency, is known as the Curie Point Temperature (hereinafter referred to as T.sub.C). On the low temperature side of T.sub.C (i.e., the "A" portion of FIG. 1) the material is ferroelectric (i.e., the material is non-isotropic). When the material is ferroelectric, it typically has a tetragonal or rhombohedral crystal structure and is capable of being poled. Thus, individual domains within the material will attempt to align themselves with an externally applied electric field causing the otherwise randomly oriented dipoles of the polycrystalline material to be oriented. Such orientation results in a net distribution of positive and negative charges in the polycrystalline material (i.e., a dipole). Piezoelectric materials can then be utilized for such applications as transducers for sound (e.g., microphones, alarm systems), high power ultrasonic generators (e.g., sonar, ultrasonic cleaning), pick-ups and sensors (e.g., record players), resonators and filters (e.g., radio, television), and the like.
However, on the high temperature side of T.sub.C (i.e., the "B" portion of FIG. 1), the material is paraelectric (i.e., the material is isotropic). When the material is paraelectric, it typically has a cubic crystal structure. It is the paraelectric phase of particular ceramic compositions which is the focus of this application.
Lead magnesium niobate (hereinafter referred to as PMN) was discovered to be ferroelectric as early as 1961. In approximately 1980, the electrostriction phenomenon was discovered to exist in PMN. However, pure PMN has a T.sub.C which occurs at approximately -20.degree. C. Moreover, the dielectric constant for PMN is very temperature dependent which means that a slight deviation from the temperature at which T.sub.C occurs leads to a large change in the dielectric constant. Thus, attempts have been made to modify the pure PMN system with dopants. For example, PMN has been doped with between 8-10 mole percent lead titanate (hereinafter referred to as PT). By doping PMN with PT, thereby resulting in a PMN-PT solid solution, the T.sub.C was raised to a higher temperature and the resulting dielectric constant for the PMN-PT solid solution was higher than the dielectric constant of pure PMN. Thus, the strain which can be developed in the PMN-PT system is higher than the strain which can result in the pure PMN system because the dielectric constant for the PMN-PT system is higher at T.sub.C. Accordingly, because strain is proportional to the square of the dielectric constant in an electrostrictive composition, a small change in dielectric constant can have a dramatic impact on the resultant strain in the material. Thus, PMN doped with 8-10 mole percent of PT results in a material which has a T.sub.C closer to room temperature. This shifting of T.sub.C is very important for the following reasons.
If an electrostrictive material is to be used at room temperature, it is desirable for that material to exhibit a T.sub.C at or near room temperature. This permits maximization of the dielectric constant and thus maximization of strain. However, for many applications a constant environmental temperature, such as room temperature, cannot be assured. Thus, in reference to FIG. 1, it is clear that for known electrostrictive materials the dielectric constant is largely dependent upon temperature. Accordingly, all previously known applications for electrostrictive materials have been restricted to environments which have a substantially constant temperature. However, if it is desired to use electrostrictive materials for applications in which constant environmental temperatures cannot be assured, then the known prior art compositions are inadequate.
For example, in reference to FIG. 1, if the environmental temperature to which the electrostrictive material is exposed changes as little as 20.degree. C., the dielectric constant could be reduced in value by 1/3-1/2. This would be undesirable from an application perspective because such changes in dielectric constant would result in fluctuating measurements. Alternatively, the calibration or control devices connected to the electrostrictive material would have to be very complex to compensate for such fluctuations in dielectric constant, otherwise, any measurements taken would be unreliable. In other words, all known electrostrictive compositions suffer from the drawback that even if a high dielectric constant can be achieved for a particular temperature, if the environmental temperature varies slightly from T.sub.C, then, the dielectric constant also varies resulting in an unreliable system.
There are also known various manufacturing processes for the formation of an electrostrictive ceramic material. Such processes include the conventional mixing of oxides. However, as is the case in many electroceramic compositions, many investigations continue in an effort to determine an optimum process for forming electrostrictive ceramic materials.